The differentiation of rat myoblast cultured cells, L6A1R5, is stimulated by insulin and is accompanied by an increase in creatine kinase (CK) activity. Creatine kinase mRNA was measured by hybridization with creatine kinase cDNA. A dose of insulin that caused a 203 fold increase in CK activity produced only a 17.5 fold increase in CK mRNA. This suggests that insulin has two effects, an increase in mRNA levels and an additional effect on the rate of protein synthesis or processing.